Oh yeah, I forgot to correct my previous post about this. DreamweaverMX is the version I've tested this on and it works for me, but I doubt it will work the same way in Dreamweaver 3.x or UltraDev.
However, it only works if you have the file open, then add the DOCTYPE code, then save the file, but don't close it. You should then see the code rendered in design view.
After you close the file, if you want to edit it visually again, you simply need to open the page and make some kind of minor edit to it (I usually just add a space somewhere) then resave it and the display should refresh with the rendered code. You'll have to do that each time you re-open the file if you want to view in design view. Sorry for the confusion.
Getting back into x-cart dev after a long hiatus. Modded lots of x-carts from version 3.1.x to 4.1.x. Developer of ImageScaler mod, Pre-login per user coupon mod, Wordpress feed mod, DigitalSubscriptions mod, Phonetic bulk download keys addon for DownloadExpander mod, Serial Number Generator for ESD products, Custom CMS/LMS integrations, external products mod, and more.